China said on Thursday that "arbitrary" tariffs such as the 50 percent levy on copper announced by US President Donald Trump on national security grounds "serve no party's interests".

"We have always opposed the overstretching of the concept of national security," Foreign Ministry spokeswoman Mao Ning said.

"We have also always believed that there are no winners in tariff wars and trade wars, and the arbitrary imposition of tariffs serves no party's interests."

Trump announced on Wednesday that a US "national security" levy of 50 percent on imports of copper, a key metal for green energy and other technologies, would come into effect on August 1.

Copper demand has surged in recent years as the world transitions to cleaner energy, with the metal needed for solar panels, wind turbines and electric-vehicle batteries.

The International Energy Agency forecasts that global copper consumption will increase by more than 25 percent to 33 million tonnes by 2035 from 26 million in 2023.

Trump's aggressive trade blitz has rattled global markets since his return to office in January.

He ordered an internal assessment in February on whether limited copper imports posed a threat to US national security.

Trump later promised to implement tariffs on copper, mirroring similar action on other key metals aluminium and steel, driving up prices as US buyers built up reserves. (AFP)